\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}a \times (a+1) \times (a+2) \times \ldots \times (a+n) = \frac{(a+n)!}{(a-1)!}\end{equation*}
\end{document}
